In recent years, frequently occurring climate changes and natural disasters, which are seemingly attributable to global warming, have had a significant impact on agricultural production, the living environment, energy consumption, and the like. The global warming is believed to be due to the increase in greenhouse gases, typically CO2, in the atmosphere, resulting from intensive human industrial activities. Therefore, there is an urgent demand for a measure to lower the atmospheric concentrations of CO2.
Major sources of CO2 include thermal power plants, boilers of factories, kilns of cement factories using coal, heavy oil, natural gas, or the like, as a fuel, blast furnaces of ironworks where iron oxide is reduced with coke, and transportation equipment, such as automobiles, marine vessels, aircraft, and the like, using gasoline, heavy oil, light oil or the like, as a fuel. Except for transportation equipment, these sources of CO2 are fixed facilities, and are expected to be easily adapted to implementation measures for reducing CO2 emissions into the atmosphere.
A wide variety of methods for recovering CO2 from gases exhausted from the above-mentioned sources have been studied, and several methods are known.
For example, a method for absorbing CO2 by bringing an aqueous solution of an alkanolamine into contact with a CO2-containing gas in an absorption tower is well known. Examples of known alkanolamines include monoethanolamine (hereinafter, sometimes referred to as “MEA”), diethanolamine (DEA), triethanolamine (TEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA), and diglycolamine (DGA). MEA is typically used.
However, primary amines, such as MEA, are highly corrosive to device materials, and therefore the use of an aqueous solution of such an alkanolamine as a solution for absorbing CO2 requires the use of expensive, corrosion-resistant steel, or requires lowering the concentration of the amine in the absorbing solution. Further, although absorbed CO2 is typically released and recovered in a regeneration tower by heating the solution to a temperature of about 120° C., this method ends up consuming a large amount of energy for recovery per unit weight CO2 because the use of the above-stated alkanolamines is unsatisfactory in terms of the amount of absorbed CO2 in an absorption tower and the amount of released CO2 in a regeneration tower.
At the present time, where the reduction of CO2 emissions and the saving of energy and natural resources are being sought, a significant amount of energy consumption for the absorption and recovery of CO2 is an obstructive factor to the practical use of the aforementioned technique. Thus, a technique for separating and recovering CO2 with less energy is desired.
As an example of prior art techniques for separating and recovering CO2 by using less energy, Patent Document 1 discloses a method for removing CO2 from a combustion exhaust gas by bringing an aqueous solution of a so-called hindered amine, which has a steric hindrance of alkyl groups or the like around the amino group, into contact with a combustion exhaust gas at atmospheric pressure to allow the aqueous solution to absorb CO2.
In Patent Document 1, 2-methylaminoethanol (hereinafter, sometimes referred to as MAE) and 2-ethylaminoethanol (hereinafter, sometimes referred to as EAE) are described as a hindered amine, and 30 wt % aqueous solutions of MAE and EAE are used in the Examples. Other examples of hindered amines, although not used in the Examples, include amines, such as 2-(isopropylamino)ethanol (hereinafter, sometimes referred to as IPAE).
Patent Documents 2 to 6 disclose absorbing solutions containing N,N,N′,N′-tetramethyl-1,3-butanediamine, or N,N,N′,N′-tetramethylhexane-1,6-diamine, and methods for removing CO2 by using the absorbing solutions.